The invention relates generally to a method and apparatus for synchronizing to pulse generating circuitry, and in particular, to a method and apparatus for synchronizing a high frequency pulse generating circuitry to packets of lower frequency pulse signals derived from a scanning system which traverses a ruled grating.
In one high precision scanning system, a scanning galvanometer traverses a beam provided from a laser source across an object to be scanned. Simultaneously, the laser-generated energy beam scans across a ruled grating to provide accurate positioning information for the scanning system. In a typical system, the ruled grating provides substantially coarser position markings (typically eighty line pairs per inch) than required by, for example, a precision recording and reading system. Since the coarse grating provides insufficient precision for the system, the scanning apparatus synchronizes a higher frequency pulse train to the grating derived pulse train and outputs a second pulse train which can have, for example, 100 or more pulses between each of the ruled grating derived pulses.
As the scanning equipment becomes more sophisticated and precise, it is desirable to provide yet greater performance precision for the system. For example, the system can be employed to effectively magnify a section of the region being scanned, and in this mode of operation it is desirable to effect changes in system magnification of one part in a thousand. In order to achieve this precision, it is necessary to output, based upon the ruled grating markings, a pulse train which can be varied by one part in a thousand.
This presents a substantial difficulty to a standard synchronization system which typically operates in a phase locked loop configuration. The problem results first from the nonperiodicity of the pulse train (the scanning beam moves fastest at the middle of its scan and has the greatest rate of change at the end of the scan) and second, because of the large dynamic range required for implementing the standard phase locked loop configuration to achieve a 0.1 percent precision. Accordingly, the standard phase locked loop would need to operate at a 20 megahertz frequency rate, a rate which would require not only specialized and precision components, but one which is practically unavailable with present day technology. If available, the cost of the system for a typical scanning galvanometer controlled apparatus, such as that used in a document reading and recording, would be prohibitive.
The scanning problem is further exacerbated by the requirement, when writing or reading a scanned document, for starting the generation or collection of data at the same sample position along each scan line. If this is not done, and it is especially objectionable in a written document, there results an initial ragged edge and a resulting lack of synchronization from line to line of the scanned material. This important requirement is further complicated by the varying scanning velocity of the galvanometer along its scan path (resulting in a change in the time duration between pairs of grating lines along the ruled grating) and the desire to "magnify" different portions of a scanned document or to write different sized documents using different "magnifications."
It is therefore a primary object of the invention to provide an economical, simple, reliable, and repeatable synchronization apparatus and method which provide a 0.1 percent step size in scanning magnification and a high repeatability of scan starting location. Other objects of the invention are a scanning apparatus and method which can be constructed using standard circuit components.